The more components to a ROV system the more complex the console arrangement becomes. It is also interesting to note that the system hardware and software are of varying ages. For instance, the Trackpoint technology is at least 30 years old. The ROV on-screen display that displays video and bearing information is also fairly antiquated, but like Trackpoint is robust technology. The on-screen display can also display text, allowing for meta-data to be recorded for each dive.
Due to the need to accurately plot the position of the ROV on the bottom the console arrangements for the abalone project are particularly complex, leading to the need for prior technical knowledge. To ease this reliance on personal knowledge the console components and arrangement are documented and diagrammed (Figure 1) so that set up and break down processes can be successful, even with inexperienced staff. A component list of the console equipment and software is provided in Appendix II.
3.2.1 Directional Hydrophone, Transponder and Trackpoint
By mounting a directional hydrophone on the side of the research vessel and a transponder on the ROV, surface tracking of the ROV during dives is possible. The transponder is located on the top of the ROV and needs to be activated before each dive. It “pings” at a rate of once every two seconds. The output data from these instruments are initially displayed on the Trackpoint monitor plotting the location of the ROV in relation to the relative position of the boat. This data is also sent to the ROV tracking program, WinFrog, for display on the computer monitor, hence the Trackpoint monitor provides a built-in redundancy in case of computer malfunctions. These data are crucial to launch, retrieval and piloting of the ROV as the ROV can never stray either too far from the vessel or too close to the propellers.
When moving between sites the directional hydrophone can remain bolted into position as long as the vessel does not exceed a speed of approximately eight knots.
Positional data from a Differential Global Positioning System (DGPS) located on the surface are used along with the positional information obtained from the hydrophone/transponder in WinFrog to determine the actual location of the ROV. The distances from the DGPS antenna (ideally located near the center of the ship) to the bow and stern of the ship are measured and offsets are entered. This information, along with the directional hydrophone/transponder data collected by the Trackpoint system, is used to determine and display the precise location of the ROV in relation to the ship at any given time. The estimated error of the current DGPS system is approximately 3 m. For accurate mapping of the ROV position onto both the WinFrog display and later into a geographic Information System (GIS) it is important that the datum used in the DGPS is known (the current system uses WGS 84). The latitude and longitude data from a particular dive (displayed as a track line) are exported from the WinFrog program and stored as permanent maps using ArcView GIS (ESRI, 2004).
3.2.3 Pitch and Roll Sensor
On a large vessel such as the David Starr Jordan, pitch and roll can be in the order of 20 – 30 meters in moderate seas, which means the DGPS positions also waiver by this distance, even if the ROV is traveling in a straight line. If estimates of ROV transect area are required this error needs to be removed. The pitch and roll sensor is a large gyroscope that provides data to the Trackpoint system to adjust the range and bearing to the ROV for the movement of the ship and correspondingly movement of the directional hydrophone. Like the directional hydrophone, the pitch and roll sensor needs to be correctly orientated, but in this case it is bolted onto the ship along its midline. It is important that the offsets of the hydrophone, GPS antennas and pitch and roll sensors are accurately entered into the WinFrog program.
3.2.4 Tracking and Data Processing Software- WinFrog & Ribbit
The positional information from the directional hydrophone, pitch and roll sensor data, DGPS and the ROV’s flight data are all processed and displayed in the program WinFrog (Thales Geosolutions, 2004). The graphical display is used both for navigation of the ROV and to log dive data (Fig 2). For navigation the program can display both the ROV and ship sizes and positions relative not only to each other, but also to a geo-referenced background such as a bathymetry map. The display is simultaneously relayed to the bridge, allowing the helm to adjust the speed of drift of the research vessel to maintain position along a transect line. All data are saved in various file formats (see Appendix IX). The file types generated from WinFrog are .DAT, .LOG, and .RAW files. Comma delimited .DAT files are automatically created from the .RAW files, and include some of the main measurements of interest, including position, depth, and altimeter data. The .RAW files include all data collected during the dive, and can be viewed and saved in the ascii file format in the WinFrog sister program, Ribbit. WinFrog can also record text as .LOG files which allow for events of interest (such as the discovery of an abalone) to be logged manually during the dive. WinFrog has many other options that are very useful in ROV operation, including map overlays and distance calculations. There are numerous settings that must be properly adjusted prior to successful use of the program, so it is recommended that a sufficient understanding of the program be reached well before cruise time (see Appendix VIII).
3.3 The Tether
The tether secures the ROV to the boat and delivers power and commands from the console to the ROV via the junction box. It also carries images and data from the ROV, through its umbilical connection, and back up the cable to the console. To fulfill this task it contains 32 small wires bundled and sealed within plastic sheaths. Due to this complexity and the fragility of the wires, it is the weakest link of the ROV system. Tether care and management is crucial to cruise success. To maintain the integrity of the tether there should be no bends less than 2 feet in diameter, padding should be placed at any friction points, floats should be attached to the tether near the ROV, and nothing heavy should be placed onto the tether. It is also of great benefit to have a tether real to coil and uncoil the tether. Effective on deck tether management is vital to maintaining the integrity of the tether.
Never reach the end of your tether. There are two ends of the tether, the end where the tether is attached to the boat, and the clump weight end. If the tether reaches either end, stress is placed onto the umbilical and the ROV pilot loses control of the vehicle and any extra length of tether that may be necessary to avoid problems.
3.4 Vessel and Cranes
ROV operations, even to a greater extent than most work at sea, are highly weather dependent. Surface conditions must allow the support vessel to maintain its position above the ROV, and subsurface conditions must also allow the ROV to maintain its position under the support vessel. The horse power of the ROV motors determines the speed through the water and is an important factor in whether operations are successful in the case that conditions are unfavorable. For the Phantom DS4 both surface drift and subsurface currents of less than 2.5 knots are required in order for safe and effective ROV operations to take place. The operation of the ROV is highly dependent on the vessel speed. If the vessel moves too fast across the bottom (> 1 knot) depending on the specific survey technique, it can be very difficult for the ROV pilot to keep the vehicle near the vessel, and in turn impede collection of realistic data. The skill of the helm to maintain slow speeds along a predetermined transect in variable wind and current conditions cannot be under-emphasized. For the abalone study, a ship speed across the bottom of ½ knott is ideal, although not always possible in certain conditions.
A crane is used to maneuver the ROV while it is on deck and at the surface of the water. Clear instructions and communication are needed between the vessel crew and research scientists for smooth ROV operations. The crane operator must be in excellent communication with the scientists, the ROV pilot, the clump weight tender, and the helm (Appendix VI).
A 350 lb clump weight is used to minimize drag on the ROV tether cable. The weight is attached to a wire cable and lowered over the side of the vessel to a depth of 5 m. The ROV, after being flown 20-40 m away from the boat, is then joined by the tether to the wire by means of straps and carabineers. Both the tether and the wire are then lowered together and additional carabineers are attached at 10 m intervals. When lowering the weight a cable counter is used to determine the length of cable deployed. The first connection is 5 m above the clump weight and includes a protective plastic sleeve around the ROV cable, as this is the point of greatest strain. The length of “flying” tether is determined by the substrate conditions. In complex rocky terrain shorter lengths are used to minimize the chance of entanglement, while in habitat comprised of simple sandy bottoms longer lengths allow for greater freedom of movement. In strong currents longer lengths of tether may be needed to navigate down to the bottom.
Cable counter and depth sounder displays are useful to have in the laboratory, as the position of the weight should generally be maintained 10 m above the bottom. It is imperative that a weak link is established in the cable so that the vessel will not be dragged under if the clump weight snags onto the bottom.
4. Habitat Mapping and Transects
4.1 Habitat Mapping and Transects
When sampling is focused on specific habitats, bathymetric mapping prior to ROV surveys provides very useful information. Multi-beam sonar techniques are used to produce detailed contour maps of the bottom, with bathymetric relief used as a surrogate for rocky habitat type (Figure 1). Once a site has been selected the ship steams to a position up-wind from the likely habitat. The ROV is deployed to the windward side, dived to the habitat and a transect begins (Appendixes III – V). The helm then maneuvers the ship along a bearing that travels along the depth stratum and allows the ROV to maintain contact with the rocky habitat (Appendix X).
Each replicate is a strip transect of approximately 1-2 km in direct length with a field of view determined by the speed, height off the bottom, position, pitch, and roll of the ROV. The vehicle flies between 0.5- 1 m above the substrate along each transect, following the middle contour of a single depth strata. Transect tracklines of the ROV across the substrate are plotted for each dive on a map.